Information Notice No. 97-73: Fire Hazard in the use of a Leak Sealant

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
September 23, 1997
NRC INFORMATION NOTICE 97-73: FIRE HAZARD IN THE USE OF A LEAK SEALANT
Addressees
All holders of operating licenses for nuclear power reactors except those who
have permanently ceased operations and have certified that fuel has been
permanently removed from the reactor vessel.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice
to alert addressees to a potential fire hazard in the use of a leak sealant. It
is expected that recipients will review the information for applicability to
their facilities and consider actions, as appropriate, to avoid similar problems.
However, suggestions contained in this information notice are not NRC
requirements; therefore, no specific action or written response is required.
Description of Circumstances
Discussion
On July 15, 1997 a steam leak in a main steam isolation valve (MSIV) at Unit 2
of the South Texas Project plant was being repaired by injecting a leak sealant.
The people performing the work had reached a stopping point to let the injected
compounds firm up to create the seal. The work team left the cubicle in which
the work was being performed in order to cool off, the temperature in the cubicle
being too high to allow them to stay there. A patrolling security guard entered
the cubicle and noticed a small flame on the valve. Appropriate personnel were
notified and the flame was put out with a fire extinguisher.
The compounds used to seal the leak were approved products for the application
and are in common use in the industry. Mineral oil is one of the material
components of the compound. When the compound is injected under pressure and
gets hot, the mineral oil tends to separate from the rest of the compound,
becoming more fluid and starting to leak out wherever it can find a path. The
leakage may be either as a liquid or as a vapor (smoke) due to the high
temperature.
In this event, the mineral oil leaked out from the sealant-retaining clamp as a
liquid and soaked into the fiberglass insulation around the valve. The "wicking"
effect from the penetration into the insulation caused the auto-ignition
temperature of the mineral oil to be
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September 23, 1997
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lowered sufficiently to ignite the oil and produce small flames on the insulation
pad. Porous materials, such as blanket-type insulation, exhibit a wicking effect
when exposed to a combustible liquid. When the impregnated insulation is exposed
to elevated temperatures, the combustible liquid (in this case mineral oil) may
spontaneously ignite even though the temperature is below the published auto-
ignition temperature of the combustible liquid. An outside ignition source is
not required for combustion to occur in oil-soaked insulation.
Auto-ignition temperature is the temperature at which combustion will occur
without an outside ignition source. The flash point is the lowest temperature
at which the vapor of a combustible liquid can be made to ignite momentarily in
air; i.e., will burn as long as an open flame is present and will not continue
to burn if the flame is removed. Wicking action actually lowers the surrounding
vapor pressure which results in the lowering of the auto-ignition temperature.
Spontaneous ignition of oil-soaked insulation can occur under the following
conditions:
- the liquid is insufficiently volatile to evaporate rapidly,
- the insulation is sufficiently porous to allow oxygen to diffuse to the
surface of the absorbed liquid, and
- the oil leak is slow enough that the pores of the insulation are not
blocked so that oxygen is not excluded from the high-temperature region.
The South Texas Project licensee has taken steps to preclude repetition of this
event. Revisions to engineering standards incorporate information about sealing
compounds with greater-than-minimal percentage of mineral oil and appropriate
temperature limitations. Work instructions related to on-line leak repair
activities contain requirements to:
- ensure that insulation and other materials that may cause a wicking effect
are removed from the work area,
- ensure that a fire watch is established during work activities and for an
hour after work is complete, and
- provide adequate ventilation to assist in disbursement of vapors.
This information notice requires no specific action or written response. If you
have any questions about the information in this notice, please contact one of
the technical contacts listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
signed by D.B. Matthews for
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Geoffrey P. Hornseth, NRR Joseph I. Tapia, Region IV
301-415-2756 (817) 860-8243
E-mail: gph@nrc.gov E-mail: jit@nrc.gov
Attachment: List of Recently Issued NRC Information Notices